Ball bearing

ABSTRACT

A rolling bearing composed of a combination of bearing wings formed with a steel material and rolling elements formed with a ceramic material, wherein the residual austenite amount of the steel material forming the rolling rings is controlled to a specific value. By the construction, at rolling of the rolling elements formed with a relatively hard ceramic material, the occurrence of the surface scratches of the rolling elements can be restrained, which results in restraining the vibrations and noises caused by the rolling bearing.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a Continuation-In-Part of earlier U.S. patentapplication Ser. No. 09/122,898 filed on Jul. 24, 1998.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to roller bearings, and moreparticularly to a roller bearing suitably used, for example, forinformation instruments such as hard disc devices (HDDs), video taperecorders (VTRs), laser printers and the like.

[0003] Previously, roller bearings for supporting a spindle motor ofhard disc devices and the like have employed a bearing ring and rollingelement both of which are formed of steel.

[0004] In the above-described hard disc devices, to prevent theoccurrence of writing errors and reading errors, the improvement of theacoustic characteristics such as vibrations, noises, etc., has beenrequired.

[0005] Also, because these information instruments are frequently placednear users at a relatively calm place in an office, a home, etc., it isstrongly desired to reduce the generation of noises, particularly,having the frequency in the audible range of a human.

[0006] Under such circumstances, to quiet a rolling bearing forsupporting a spindle motor, it has been conventional practice to reducethe shape errors of each bearing component as much as possible, forexample, by maximizing the roundness of inner and outer rings and byequalizing the sizes of all rolling elements.

[0007] In addition, when the constituting elements of a rolling bearingare formed of ball bearing steel, a heat treatment such as hardening,tempering, etc., is applied to the ball and ring elements which resultsin the residual austenite in the rolling element and ring beingapproximately 10%.

[0008] When such conventional roller bearings are subjected toinadequate lubrication, overheating results so as to cause defects suchas fine impressions, cracks and peelings to form on the roller elementsand the bearing rings, with such defects resulting in undesirable noiseand vibration.

[0009] Also, as the result of investigations, it has been confirmed thatone of the causes of noises generated by a rolling bearing is the finescratches formed on, in particular, rolling elements. On the other hand,recently developed roller elements formed of hard ceramic material areresistant to being scratched. When the rolling element is formed with aceramic material, the generation of noises by scratches is reducedbecause the ceramic material of the roller differs from the steelmaterial of the bearing ring; also, sticking of the ball and ringcomponent by coagulation because of inadequate or inferior lubricant canbe reduced.

[0010] As described above, the formation of rolling elements with aceramic material is effective for improving the acoustic characteristicsof a ball bearing. However, the inventor's further investigationsrevealed that use of ceramic rollers creates a further problem needingto be solved.

[0011] That is, when a rolling element is formed of ceramic, because thehardness and rigidity of the rolling element is increased and therolling element is hard to deform, the contact area of the rollingelement and the bearing ring becomes small and the stress applied to theorbital surface becomes large. Accordingly, when the bearing ring isformed of conventional steel wherein the residual austenite amount isabout 10%, the contact portion with the rolling bearing element isliable to cause permanent ring deformation and resultant generation ofunacceptable vibrations and noises. Also, when stress applied to theorbital surface becomes large, there is a problem that austenite isliable to cause a martensite transformation. Furthermore, in the case ofusing a ball bearing steel wherein the residual austenite amount isabout 10%, there is a problem that the amount of the generated heat bythe martensite transformation increases heat generation and undesirableexcessive bearing temperature.

SUMMARY OF THE INVENTION

[0012] Accordingly, an object of the present invention is to provide arolling bearing providing a reduction in undesirable characteristicssuch as vibrations, noises and the like.

[0013] Another object of the present invention is to provide a rollingbearing capable of stable and continuous excellent performance for along period of time even under operating conditions of severe lubricantdeficiency.

[0014] Still other objects, the features, and the merits of the presentinvention will become clear from the following descriptions.

[0015] That is, a first aspect of the present invention is a rollingbearing comprising a bearing ring formed with a steel material and arolling element formed with a ceramic material, wherein the residualaustenite amount of the steel material forming the bearing ring is notmore than 8%.

[0016] Furthermore, a second aspect of the present invention in apreferred embodiment is a rolling bearing of the first aspect whereinthe residual austenite amount of the steel material forming the bearingring is from 0.05% to 6%.

[0017] In addition, if the residual austenite amount (Y_(R)) is lessthan 0.05%, there is an increased tendency for the formation of cracksso as to decrease the life expectancy of the bearing. On the other band,if the residual austenite amount exceeds 8%, the hardness is decreasedso as to permit increased permanent deformation of the rolling bearingsurface by either static loads or impact loads causing unacceptablescratches and impressions in the rolling bearing surface. The criticalvalue of the residual austenite (Y_(R)) is consequently specified asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other objects as well as advantages of the presentinvention will become clear by the following description of preferredembodiments of the present invention with reference to the accompanyingdrawings, wherein:

[0019]FIG. 1 is a cross-sectional view of the upper half of a preferredembodiment of the rolling bearing of the present invention;

[0020]FIG. 2 is a graph showing the change of the vibration of therolling bearing of FIG. 1 with the passage of time; and

[0021]FIG. 3 is a cross-sectional view showing an using example of therolling bearing of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0022] Then, the preferred example of the rolling bearing of the presentinvention is explained by referring to the accompanying drawings.

[0023] First, a prefer-red example of the rolling bearing of the presentinvention is explained by referring to FIG. 1 to FIG. 3.

[0024] The rolling bearing A according to the preferred example of thepresent invention is a deep groove ball bearing. The rolling bearing Ais constituted by an inner ring 1, an outer ring 2, plural rollingelements 3, and a retainer 4. Also, the inner ring 1 and the outer ring2 are formed with a metal material, the rolling elements 3 are formedwith a ceramic material, and the retainer 4 is formed with a syntheticresin material, etc.

[0025] Practically, the inner ring 1 and the outer ring 2 are preparedby, after shaping the forms using a ball bearing steel (JIS Standard SUJ2), applying a series of heat treatments such as a hardening treatment,a sub-zero treatment, a tempering treatment, whereby the residualaustenite amount (Y_(R)) is controlled to become from 0.05 to 6%, andthereafter, by applying thereto a finishing treatment such as polishing,etc. The above-described hardening treatment is the treatment of holdingfor several tens minutes at 850° C. and thereafter oil-cooling. Thesub-zero treatment is the treatment of holding for from several tensminutes to several hours at −70° C. and thereafter air-cooling. And thetempering treatment is the treatment of holding for several tens minutesto several hours at 220° C. and thereafter air-cooling. For example,when in the hardening treatment, after holding for 10 minutes at about850° C., oil-cooled, in the sub-zero treatment, after holding for 1 hourat −70° C., air-cooled, and in the tempering treatment, after holdingfor 1 hour at 220° C., air-cooled, the residual austenite amount (Y_(R))can be controlled to 2%. The region of the residual austenite is asurface portion, for example, is the portion from the surface to a depthof about 10 μm, and also the surface hardness is established to be from60 to 64 in HRC. In addition, as the material for the inner ring 1 andthe outer ring 2, in addition to the above-described material, JISstandard SUS440 and various ball bearing steels obtained by improvingthe above-described JIS standard SUJ2 may be used.

[0026] The rolling elements 3 can be formed with ceramic material madeup of silicon nitride (Si₃N₄) as the main body and using yttria (Y₂O₃)and alumina (Al₂O₃), and further aluminum nitride (AlN), titanium oxide(TiO₂), and spinel (MgAl₂O₄) as sintering aids, and also can be formedwith ceramics using alumina (Al₂O₃), silicon carbide (SiC), zirconia(ZrO₂), aluminum nitride (AlN), etc.

[0027] Practically, it is preferred that the rolling elements 3 areceramic balls formed with a ceramic made up of from 1.5 to 5.5% weightyttria (Y₂O₃), from 1 to 2% by weight aluminum nitride (AlN), from 2 to4.5% by weight alumina (Al₂O₃), from 0.5 to 1.0% by weight titaniumoxide (TiO₂), and rest being silicon nitride (Si₃O₄). Also, when therolling element 3 is a ball having a diameter of 2 mm, it is preferredthat the precision is JIS Standard B1501-G3 or higher, for example, thedesign shown in following Table 1. It should also be noted that JISStandard B1501-G3 or higher, is applicable to balls of any diameterincluding, but not limited to, balls having diameters ranging between0.8 mm to 2.7781 mm ({fraction (7/64)} inch). TABLE 1 SURFACE DIAMETERDIAMETER ROUGHNESS DIFFERENCE UNEQUAL SPHERICITY Ra OF ROTS CLASSLARGEST LARGEST LARGEST LARGEST 3 0.03 0.03 0.002 0.05 5 0.08 0.08 0.0040.14

[0028] It has been determined that ceramic balls 3 which range indiameter from approximately 0.8 mm to approximately 2.7781 mm andspecific diameters include 0.8 mm, 1.2 mm, 1.5875 mm ({fraction (1/16)}inch) and 2.7781 mm ({fraction (7/64)} inch) provide equally desirableresults. However, the invention is not limited to the aforementioneddiameters since both smaller and larger sizes than those noted shouldprovide equally impressive results. The standards and accuracy reflectedby Table 1 are equally applicable to different diameter balls of alldiameters including the foregoing recited diameters.

[0029] The retainer 4 can be formed with a general polyamide resin(nylon 66) or other thermoplastic resins having a heat resistance, forexample, fluorine-based resins such as polytetrafluoroethylene (PTFE),ethylene tetrafluoroethylene (ETFE), etc., and engineering plastics suchas polyether ether ketone (PEEK), polyphenylene sulfide (PPS), polyethersulfone (PES), nylon 46, etc. In addition, in the use under alow-temperature circumstance at about room temperature and under aright-load condition, polypropylene (PP) or polyethylene (PE) can beused. Also, as the form of the retainer 4, in addition to the crown formshown in FIG. 1, a wave form, etc., is optionally used.

[0030] In the above-described rolling bearing A according to thepreferred practical examples of the present invention, the orbitalsurfaces of the inner ring 1 and the outer ring 2 have a proper hardnessand also a proper tenacity and have a stable structure that the residualaustenite is hard to cause a martensite transformation. Also, the workscratches on the surface of the rolling element 3 are less than those ofa steel-made rolling element and the rolling movement of the rollingelements 3 becomes smooth.

[0031] In particular, when the rolling elements 3 are formed with aceramic, the contact surface pressure becomes large but by constructingthe inner ring 1 and the outer ring 2 as described above, fineimpressions, cracks, peelings, etc., are hard to form on the inner andouter rings 1 and 2, whereby the occurrence of the surface scratcheswith the passage of time is restrained.

[0032] Moreover, because the material of the inner and outer rings 1 and2 differs from the material of the rolling elements 3, even in thecircumstance of severe lubricant quality or quantity conditions, and ofthe rolling elements 3 coming into rolling contact with slipping,surface scratches of the inner and outer rings 1 and 2 and the rollingelements 3 resulting from sticking by aggregation as in the conventionalcase of using the same metallic material for these parts, becomesunlikely to occur. That is, the scratch-resistance effect of the innerand outer rings 1 and 2 and the rolling elements 3 is greatly improvedas compared with the conventional cases and also the dimensional changesof the inner and outer rings 1 and 2 are restrained, whereby the rollingmovement of the rolling elements 3 can be smoothly maintained.

[0033] From the above-described improvements, the acousticcharacteristics such as vibrations and noises of the rolling bearing Acan be stabilized for a long period of time at a low value.

[0034] The acoustic characteristics of the rolling bearing A aspractically determined are explained using FIG. 2. Although not shown inthe figure, the test was carried out in the state that an axis issupport in the inside wall of an outer cylinder via a pair of two samplebearings. As the above-described bearings, the roller bearings describedabove as the preferred practical example were used and as a comparativeexample, rolling bearings wherein the inner and outer rings and therolling elements were formed with a ball bearing steel which wassubjected to a hardening treatment and a tempering treatment were used.

[0035] The sample bearing is call no. 695 of JIS Standard, thepressurization is 1.5 kgf, and the outer ring rotation is 72 rpm. Thelubricating state is an oil dropped lubrication (1 mg, viscosity 14 cstat 40° C.).

[0036] The acoustic value is almost the same at the initial value in thepreferred example of the present invention and the comparative examplebut in the case of the comparative example, the acoustic value rapidlyincreased after 800 hours and in the case of the preferred example ofthe present invention, the test was stopped after 1400 hours but thevalue is almost the same as the initial value. As described above, inthe preferred example of the present invention, the acousticcharacteristics such as vibrations and noises can be stabilized for along period of time at a low value.

[0037] The rolling bearing A according to the preferred example of thepresent invention described above can be used for supporting a spindlemotor of a hard disc device as shown in FIG. 3. In FIG. 3, the numeral50 shows a spindle motor for HDD and a rotor hub 51 to which a discs 53are fixed is rotatably supported via a pair of rolling bearings A, A toa motor spindle 52 to which a stator 51 is fixed.

[0038] In this case, an excellent calmness can be maintained such thatthe rotary shaking of the motor spindle 52 can be reduced to as littleas possible and the occurrence of the vibrations and the noises of therolling bearings A, A can be restrained. Thus, the rolling bearing ofthe present invention can largely contribute to the prevention of theoccurrences of the writing errors of information to the disc 53 and thereading errors of information from the disc 53.

[0039] In addition, the rolling bearing of the present invention can beutilized for not only a spindle motor for a HDD but also a video taperecorder and a laser beam printer, the support of the rotary portions bythe rolling bearing is stabilized, and thus, the rolling bearings of thepresent invention can contribute to the improvement of the performance.

[0040] While there has been described what is at present considered tobe preferred embodiments of this invention, it will be understood thatvarious modifications may be made therein, and it is intended to coverin the appended claims all such modifications as fall within the truespirit and scope of this invention.

What is claimed is:
 1. A rolling bearing comprising a bearing ring madeof a steel material and a plurality of balls made of ceramic material,wherein the amount of austenite remaining in the steel material forforming the bearing ring is set to be in a range of 0.05% to 6%, and theceramic balls have a precision of at least the requirement of JISStandard B1501-G3 related to steel balls.
 2. The ball bearing of claim1, wherein the amount of austenite remaining in a portion ranging froman uppermost surface of the steel bearing ring to a depth ofapproximately 10 μm is set to be in a range of 0.05% to 6%, and whereina surface hardness of the steel bearing ring is set to be 60 to 64 unitsof Rockwell hardness HRC (corresponding to 697 to 800 when converted tounits of Vickers hardness).
 3. The ball bearing as claimed in claim 1,further comprising a retainer for holding the plurality of ceramicballs, wherein the retainer is formed of a synthetic resin material. 4.A rolling bearing comprising a bearing ring made of a steel material anda plurality of balls made of ceramic material, wherein the amount ofaustenite remaining in the steel material for forming the bearing ringis set to be in a range of 0.05% to 6%, and the ceramic balls have adiameter in the range of 0.8 mm to 2.7781 mm and a precision of at leastthe requirement of JIS Standard B1501-G3 related to steel balls.
 5. Theball bearing of claim 4, wherein the amount of austenite remaining in aportion ranging from an uppermost surface of the steel bearing ring to adepth of approximately 10 μm is set to be in a range of 0.05% to 6%, andwherein a surface hardness of the steel bearing ring is set to be 60 to64 units of Rockwell hardness HRC (corresponding to 697 to 800 whenconverted to units of Vickers hardness).
 6. The ball bearing as claimedin claim 4, further comprising a retainer for holding the plurality ofceramic balls, wherein the retainer is formed of a synthetic resinmaterial.
 7. The ball bearing of claim 4, wherein the diameter of theceramic balls is approximately 0.8 mm.
 8. The ball bearing of claim 7,wherein the amount of austenite remaining in a portion ranging from anuppermost surface of the steel bearing ring to a depth of approximately10 μm is set to be in a range of 0.05% to 6%, and wherein a surfacehardness of the steel bearing ring is set to be 60 to 64 units ofRockwell hardness HRC (corresponding to 697 to 800 when converted tounits of Vickers hardness).
 9. The ball bearing as claimed in claim 7,further comprising a retainer for holding the plurality of ceramicballs, wherein the retainer is formed of a synthetic resin material. 10.The ball bearing of claim 4, wherein the diameter of the ceramic ballsis approximately 1.2 mm.
 11. The ball bearing of claim 10, wherein theamount of austenite remaining in a portion ranging from an uppermostsurface of the steel bearing ring to a depth of approximately 10 μm isset to be in a range of 0.05% to 6%, and wherein a surface hardness ofthe steel bearing ring is set to be 60 to 64 units of Rockwell hardnessHRC (corresponding to 697 to 800 when converted to units of Vickershardness).
 12. The ball bearing as claimed in claim 10, furthercomprising a retainer for holding the plurality of ceramic balls,wherein the retainer is formed of a synthetic resin material.
 13. Theball bearing of claim 4, wherein the diameter of the ceramic balls isapproximately 1.5875 mm.
 14. The ball bearing of claim 13, wherein theamount of austenite remaining in a portion ranging from an uppermostsurface of the steel bearing ring to a depth of approximately 10 μm isset to be in a range of 0.05% to 6%, and wherein a surface hardness ofthe steel bearing ring is set to be 60 to 64 units of Rockwell hardnessHRC (corresponding to 697 to 800 when converted to units of Vickershardness).
 15. The ball bearing as claimed in claim 13, furthercomprising a retainer for holding the plurality of ceramic balls,wherein the retainer is formed of a synthetic resin material.
 16. Theball bearing of claim 4, wherein the diameter of the ceramic balls isapproximately 2.7781 mm.
 17. The ball bearing of claim 16, wherein theamount of austenite remaining in a portion ranging from an uppermostsurface of the steel bearing ring to a depth of approximately 10 μm isset to be in a range of 0.05% to 6%, and wherein a surface hardness ofthe steel bearing ring is set to be 60 to 64 units of Rockwell hardnessHRC (corresponding to 697 to 800 when converted to units of Vickershardness).
 18. The ball bearing as claimed in claim 16, furthercomprising a retainer for holding the plurality of ceramic balls,wherein the retainer is formed of a synthetic resin material.